Efforts to improve the performance of lubricant basestocks by the oligomerization of hydrocarbon fluids have been ongoing in the petroleum industry for over fifty years. These efforts have led to the market introduction of a number of synthetic lubricant basestocks. Much of the research involving synthetics has been toward developing fluids that exhibit useful viscosities over a wide temperature range while also maintaining lubricities, thermal and oxidative stabilities, and pour points equal to or better than those for mineral lubricants.
The viscosity-temperature relationship of a lubricant is one critical criteria that must be considered when selecting a lubricant for a particular application. The viscosity index (VI) is an empirical number which indicates the rate of change in the viscosity of an oil within a given temperature range. A high VI oil will thin out at elevated temperatures slower than a low VI oil. In most lubricant applications, a high VI oil is desirable because maintaining a higher viscosity at higher temperatures translates into better lubrication.
PAOs have been recognized for over 30 years as a class of materials that are exceptionally useful as high performance synthetic lubricant basestocks. They possess excellent flow properties at low temperatures, good thermal and oxidative stability, low evaporation losses at high temperatures, high viscosity index, good friction behavior, good hydrolytic stability, and good erosion resistance. PAOs are miscible with mineral oils, other synthetic hydrocarbon liquids, fluids and esters. Consequently, PAOs are suitable for use in engine oils, compressor oils, hydraulic oils, gear oils, greases and functional fluids.
PAOs may be produced by the use of Friedel-Craft catalysts, such as aluminum trichloride or boron trifluoride, and a protic promoter. The alpha olefins generally used as feedstock are those in the C6 to C20 range, most preferably 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, and 1-tetradecene. In the current process to produce low viscosity PAOs using Friedel-Craft catalysts, the dimers portion is typically separated via distillation. This portion may be hydrogenated and sold for use as a lubricant basestock, however its value is low compared to other portions of the product stream due to its high volatility and poor low temperature properties.
The demand for high quality PAOs has been increasing for several years, driving research in alternatives to the Friedel-Craft process. Metallocene catalyst systems are one such alternative. Most of the metallocene-based focus has been on high-viscosity-index-PAOs (HVI-PAOs) and higher viscosity oils for industrial and commercial applications. Examples include U.S. Pat. No. 6,706,828, which discloses a process for producing PAOs from meso-forms of certain metallocene catalysts with methylalumoxane (MAO). Others have made various PAOs, such as polydecene, using various metallocene catalysts not typically known to produce polymers or oligomers with any specific tacticity. Examples include U.S. Pat. No. 5,688,887; U.S. Pat. No. 6,043,401; WO 2003/020856; U.S. Pat. No. 5,087,788; U.S. Pat. No. 6,414,090; U.S. Pat. No. 6,414,091; U.S. Pat. No. 4,704,491; U.S. Pat. No. 6,133,209; and U.S. Pat. No. 6,713,438. ExxonMobil Chemical Company has been active in the field and has several pending patent applications on processes using various bridged and unbridged metallocene catalysts. Examples include published applications WO 2007/011832; WO 2008/010865; WO 2009/017953; and WO 2009/123800.
Although most of the research on metallocene-based PAOs has focused on higher viscosity oils, recent research has looked at producing low viscosity PAOs for automotive applications. A current trend in the automotive industry is toward extending oil drain intervals and improving fuel economy. This trend is driving increasingly stringent performance requirements for lubricants. New PAOs with improved properties such as high viscosity index, low pour point, high shear stability, improved wear performance, increased thermal and oxidative stability, and/or wider viscosity ranges are needed to meet these new performance requirements. New methods to produce such PAOs are also needed. US 2007/0043248 discloses a process using a metallocene catalyst for the production of low viscosity (4 to 10 cSt) PAO basestocks. This technology is attractive because the metallocene-based low viscosity PAO has excellent lubricant properties.
One disadvantage of the low viscosity metallocene-catalyzed process is that a significant amount of dimer is formed. This dimer is not useful as a lubricant basestock because it has very poor low temperature and volatility properties. Recent industry research has looked at recycling the dimer portion formed in the metallocene-catalyzed process into a subsequent oligomerization process.
U.S. Pat. No. 6,548,724 discloses a multistep process for the production of a PAO in which the first step involves polymerization of a feedstock in the presence of a bulky ligand transition metal catalyst and a subsequent step involves the oligomerization of some portion of the product of the first step in the presence of an acid catalyst. The dimer product formed by the first step of U.S. Pat. No. 6,548,724 exhibits at least 50%, and preferably more than 80%, of terminal vinylidene content. The product of the subsequent step in U.S. Pat. No. 6,548,724 is a mixture of dimers, trimers, and higher oligomers, and yield of the trimer product is at least 65%.
U.S. Pat. No. 5,284,988 discloses a multistep process for the production of a PAO in which a vinylidene dimer is first isomerized to form a tri-substituted dimer. The tri-substituted dimer is then reacted with a vinyl olefin in the presence of an acid catalyst to form a co-dimer of said tri-substituted dimer and said vinyl olefin. U.S. Pat. No. 5,284,988 shows that using the tri-substituted dimer, instead of the vinylidene dimer, as a feedstock in the subsequent oligomerization step results in a higher selectivity of said co-dimer and less formation of product having carbon members greater than or less than the sum of the carbon members of the vinylidene and alpha-olefin. As a result, the lubricant may be tailored to a specific viscosity at high yields, which is highly desirable due to lubricant industry trends and demands. The U.S. Pat. No. 5,284,988 process, however, requires the additional step of isomerization to get the tri-substituted dimer. Additionally, the reaction rates disclosed in U.S. Pat. No. 5,284,988 are very slow, requiring 2-20 days just to prepare the initial vinylidene dimer.
An additional example of a process involving the recycle of a dimer product is provided in US 2008/0146469 which discloses an intermediate comprised primarily of vinylidene.